Light is widely used as a medium for the transmission of information. Light has many advantages over other media, such as its independence from exterior magnetic or electric fields, its high propagation speed, large ranges, the possibility of shaping the ray beam by simple optical arrangements, etc. Furthermore, very efficient electronic components for generating and detecting light are available today. In addition, the latter distinguish themselves by a practically unlimited lifetime, a large bandwidth, and cost efficiency.
The main problem in the use of light for signal transmission is often that when more than one transmitter-receiver pair is used in the same room, each receiver may receive light from different transmitters, and the receiving device is unable to determine by which transmitter the received light has been emitted.
The light transmission from a transmitter to a receiver may take place directly or through a reflection or multiple reflections. This problem can be solved in that the light emitted by each transmitter is provided with additional information, which is evaluated and used for identification by the receivers. Many ways of achieving this are known, e.g. by associating a given wavelength and/or polarization to the light emitted by each transmitter, or by using different kinds of modulation resp. codes, as well as combinations thereof. The modulation of the emitted light is particularly interesting from the technical point of view, but it has the disadvantage of reducing the overall available bandwidth in all circumstances.
The above-mentioned problem is especially pronounced e.g. in photoelectric proximity switches. The latter are mainly used in the fields of manufacture, conveyance, and storage. Increasingly exacting demands with respect to these installations lead to a continuously increasing number of sensors in a limited space, thereby resulting in a higher probability of undesirable mutual interference between different transmitter-receiver pairs. It is therefore desirable to provide the user with transmitter-receiver pairs that can be used at will without requiring particular precautions. Transmitter-receiver pairs requiring preparatory measures in order to be individually identifiable are particularly unsuitable as they involve in particular more complicated logistics and an increased risk of errors in the installation of replacement parts.
Also, a signal connection of the different transmitter-receiver pairs is generally unacceptable because of the additional wiring.
For an improved signal-to-noise ratio and a reduced sensitivity to ambient light, photoelectric proximity switches of the prior art mainly use pulse modulated transmitting light pulses 41 as those shown in FIG. 1.
In addition, for improved switching security resp. reliability, the receiving device is designed to await a certain number N0 of received pulses before triggering a change of state of output signal 42 of the receiving device. Such a change of state signals the presence of a detected object in the monitored area. Fundamentally, this allows to reduce the risk of mutual interference between adjacent transmitter-receiver pairs as well. However, as the pulse frequency of all devices of a given manufacturing series is approximately but not exactly the same on account of component tolerances, a coincidence of the phase relation and thus an undesirable mutual interference may nevertheless occur from time to time.
German Patent No. DE 40 31 142 C 2 discloses a method where each transmitter-receiver pair ascertains whether light from an adjacent transmitter-receiver pair is currently impinging on its receiver before emitting a light pulse. If this is the case, the emission of the transmitting light pulse is delayed until the light receiver no longer receives external light, and only then is a light pulse emitted. However, this method of the prior art has several drawbacks. As the transmitting light pulses cannot be delayed for too long, the achieved operation is unsatisfactory particularly if more than two devices are used at the same time. Moreover, the operation of the method disclosed in the reference DE 40 31 142 C 2 is unreliable if transmitting light pulses are emitted by two or more devices exactly simultaneously, which is unavoidable from time to time due to the unsynchronized operation.
Other methods of the prior art make use of complicated modulations resp. codes. For example, German Patent Application No. DE 199 26 214 A 1 discloses a method where a modulation is even used in conjunction with a correlation analysis. This method of the prior art is also subject to considerable drawbacks. A major disadvantage is the time required for calculating the correlation, but also the unreliability of the obtained result. In fact, depending on the determined threshold value of the correlation function, the probability of error pulses is higher or lower while the time consumption is inversely proportional.